Each of the reactions for producing ethylene oxide or the production of phthalic anhydride or maleic anhydride is highly exothermic, involving the controlled oxidation of organic substances. It is thus necessary that the heat generated by the reaction be removed as efficiently as possible so as to prevent a run-away reaction in which undesirable products are produced and in which expensive raw materials are wasted. Accordingly, it has been the practice to utilize catalytic tubes of extremely narrow diameter. Thus, for example, it is not unusual for a catalyst tube to have an ID of 1.25 inches and to be 60 feet in length. The reactor, however, may contain as many as 2,500 to 9,000 tubes. In one instance, for example, the reactor had tubes which were only 22 feet high, but each tube had an ID of .mu. of an inch and the reactor contained 8,600 tubes. Each of the tubes are rolled or otherwise joined at each end to a tube sheet and the entire bundle of tubes and the tube sheet is jacketed and filled with a heat transfer medium, as for example, Dowtherm.TM., mercury, or molten salt solutions. Due to the heat given off by the reaction, the space velocity is maintained at an extremely high rate so that there is considerable abrasion of the catalytic spheres within the small diameter tubes. In order to maintain optimum selectivity, it is essential that the temperature be maintained within a relatively narrow range. Thus, as is pointed out by one patentee, in the case of the oxidation of ethylene to ethylene oxide, the optimum temperature for the reaction is in the range of 225.degree. C. to 250.degree. C. If the temperature falls below 225.degree. C., the conversion rate is insufficient to be economically feasible. If the temperature goes above 250.degree. C., the selectivity of the reaction for ethylene oxide decreases significantly with the concommitant loss of desired end product.
Generally speaking, it has been the practice to increase the temperature of the reaction as the activity of the catalyst begins to fade. However, as pointed out above, this has limitations, since once the temperature is increased above a certain limit, there is sufficient loss of selectivity to make the process economically unfeasible. It then becomes necessary to unload the catalyst and reload with new catalyst. Due to the high capital investment, it is essential that down time be maintained to a minimum. In most instances, turn-around time should be within less than five days.